Mass spectrometer



5 Sheets-Sheet 1 J. R. PARSONS ETAL MASS -SPECTROMETER July 12, 1960Filked April so, 1954 HMM f ym? ATTORNEYS t N m.

INVENTORS J R PARSONS D.A. FLUEGEL 5 Sheets-Sheet 2 July l2, 1960 J. R.PARSONS ETAL uAss SPECTROMETER Filed April 30. 1954 A T TOR/V575 n f A.M @le l... W. 9T.

July 12, 1960 Filed April 30. 1954 J. R. PARSONS ETAL MASS'SPECTROMETER5 Sheets-Sheet 3 D.A.FLUEGEL BY 'l' A r TORNE rs 5 Sheets-Sheet 4INVENTORS J R PARSONS D.A. FLUEGEL @ON OmN N Hicom h o@ AAAAAAA J. R.PARSONS ETAL MASS SPECTROMETER nun;

vvvvvv Ju'ly 12, 1960 Filed April 30. 1954 July 12, 1960 J. R. PARSONSETAI- MASS SPECTROMETER Filed April 30. 1954 5 Sheets-Sheet 5 All HW f@my A T TORNE YS MASS SPECTRMETER Y r.irirnes R. Parsons and Dale A.Fluegel, Bartlesville, Okla.,

Filed Apr. 30, 1954, Ser. No. 426,768

19 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometers. In one speciiic aspect itrelates to an ion velocity modulating selection type of massspectrometer. In another aspect it relates to an electron emissionregulator for ion sources. In another aspect it relates to a currentmeasuring device incorporating a servo balance system. In still anotheraspect it relates to constant output oscillators.

ln recent years mass spectronieters have been developed from highlyspecialized academic research instruments for measuring the relativeabundance of isotopes into analytical tools of extreme sensitivity andaccuracy. At the present time applications are being found for the useof mass spectromcters in process monitoring and control. Massspectrometry comprises, in general, ionizing a sample of material underinvestigation and separating the resulting ions according to theirmasses tofdetermine the relative abundance of ions of selected masses.The material to be analyzed usually is provided as a gas which isbombarded by a stream of electrons to produce the desired ions. Althoughboth positive and negative ions may be formed by such electronbombardment, most mass spectrometers make use of only the positive ions.These positive ions are accelerated out of the region of the electronbeam by negative electrical potentials applied thereto. Such potentialsimpart equal kinetic energies to ions having like charges such that ionsof different masses have dierent velocities after passing through theelectrical iield and consequently have different momenta.

The presently known mass spectrometers can be classilied into one of twogeneral groups: the momentum selection types and the velocity selectiontypes The momentum selection instruments sort the ions into beams kofdifferent masses 'by'ineans of magnetic and/or electrical deectingelds.` lons of a selected'mass are allowed to impinge upon a collectorplate to which is connected a suitable indicating circuit. The velocityselection instruments sort the ions according to the velocities impartedto the ions by electrical accelerating elds. The present invention isdirected primarily toward providing animproved mass spectrometer of thevelocity selection type.

ln United States Patent 2,535,032 there is disclosed a mass spectrometerwhich is provided with two sets of three equally spaced acceleratinggrids. Direct potentials are applied to the outer two grids and a radiofrequency potential is applied between the center grid and the two outergrids of each set. lons which enter the space between the irst two gridsin proper phase are accelerated through the fields between the first andsecond grids and the second and third grids. The ions subsequently passthrough a field-free drift space and enter the second group ofaccelerating grids. The spacing between the grids, the frequency of theaccelerating radio frequency voltage and the magnitudes of theaccelerating potentials are such that ions of predetermined mass raresPatent O 2,945,123 Patented July 12, 1960 receive sufficient energy toovercome a potential barrier and impinge upon a collector plate. Themass spectrometer of the present invention is an improvement over themass spectrometer disclosed in Patent 2,535,032. Pour sets ofaccelerating electrodes are employed to provide three separate driftspaces. This greatly irn-Y proves the resolution power of thespectrometer. The radio frequency applied to the accelerating grids ismodulated by the output of a square wave audio frequency oscillator sothat the output signal generated by the positive ions impinging upon thecollector plate is modulated at anaudio frequency. This alternatingsignal can be measured more accurately than a direct current signal. Theoutput signal is amplified by a temperature compensated tuned amplifieradapted to pass frequencies corresponding to the frequency of the audiooscillator. The

Vampliiied signal is then compared with a reference voltage and anydifference therebetween is applied to a servo motor which adjusts thefeedback in the tuned amplier, thereby varying the gain of the amplier.This adjustment of feedback is made in a manner such that the amplifiedoutput signal is equal to the reference voltage at all times. The degreeof rotation of the servov motor needed to `accomplish this equalizationis a measure of the output signal from the mass spectrometer tube.

In order that the output signal of a mass spectrometer represents thetotal ions of a preselected mass which are present in the gas sampleunder analysis, it is important to maintain the degree of ionizationconstant. is accomplished in accordance with the present invention bydisposing a screen electrode between the source of electrons and theionization chamber. The potential applied to this screen electrode isregulated in terms of the electron emission from a heated filament tomaintain a constant flow of electrons from the filament into theionization chamber.

Accordingly, it is an object of this invention to provide an improvedmass spectrometer which operates upon the principle of velocityselection of ions of a predetermined mass.

Another object is to provide a mass spectrometer of the velocityselection type wherein an accelerating potential of a tirst frequency ismodulated by a potential of a second lower frequency so that theresulting ion beam is modulated at the frequency of said secondfrequency.

Another object is to provide an electron emission regulator for an ionsource.

A further object is to provide a current measuring de.- vice whichincludes a servo system to regulate the gain of an amplifier to maintainthe output signal of the amplifier constant.

A still further object is to provide an oscillator having a voltageregulating circuit associated therewith to maintain the oscillatoroutput constant.

Various other objects, advantages and features of this invention shouldbecome apparent from the following detailed description taken inconjunction with the accompanying drawing in which:

Figure l is a schematic representation of the mass spectrometer of thisinvention;

Figure 2 is a schematic circuit diagram of the power supply circuit ofFigure A1;

Figure 3 is a schematic circuit diagram of the emission regulator ofFigure 1 Figure 4 is a schematic circuit diagram of the audio and radiofrequency oscillators of Figure 1;

Figure 5 is a schematic circuit diagram of a tuned amplifier which isincluded in the detector circuit of Fig- Ure l;

This

Figure 6 is a schematic circuit diagram of the servo balance system ofthe detector and timer of Figure l; and

Figure 7 is a schematic circuit diagram ofthe ampliiier shown in Figure6.

Referring now to the drawing in detail and to Figure 1 in particular,there is shown the mass spectrometer tube 10 which can comprise a glassenvelope, the interior of which is maintained at a reduced pressure by avacuum pump, not shown, which communicates with the interior of tube 10through a conduit 11. A sample of the gas to be analyzed is supplied bya conduit 12 which has a normally open solenoid-operated valve 13therein. A sec ond conduit 1-4 having a normally closedsolenoid-operated valve 15 therein communicates with conduit 12downstream from valve 1.3. Conduit 14 is connected to a source ofreference gas. A branch conduit 16 having a restriction thereincommunicates with conduit 12 downstream from the junction with conduit14. This conduit thus forms a viscous leak path for a sample of the gasbeing analyzed. A second branch conduit 17, having a narrow oriiice 18therein, communicates between conduit 16 and tube 10. Orifice 18 definesa molecular leak path. Valves 13 and 15 are operated in unison by atimer circuit 19. The electrical circuitry associated with massspectrometer tube 10 is energized from a source of alternating potential20, the output terminals of which are connected to a power supp-1ycircuit 21. Power supply circuit 21 is provided with a first outputterminal 22 which is maintained at a constant positive potential andwith a second output terminal 23 which is maintained at a constantnegative potential. Output terminals 24 and 25, which provide a sourceof regulated alternating current of lower voltage than source 20, areconnected to the respective end terminals of an electron emittingfilament 26 disposed in one end of tube 10. The end terminals of apotentiometer 27 are connected to respective voltage terminals 2-4 and25, and the contactor of potentiometer 27 is connected to one inputterminal of an electron emission regulator circuit 29. VA second inputterminal of emission regulator 29 is connected to negative potentialterminal 23. The electrons emitted from iilament 26 are accelerated intoan ionization chamber 30', which is de `lined by a pair of groundedspaced grids 31 and 32, by the potential diierence between grids 31, 32and iilament 26. A pair of focusing grids 33 and 34 is positioned on thesecond side of ionization chamber 30.` Grid 33 is connected to thecontactor of a potentiometer 35 and grid 34 is connected to thecontactor of a potentiometer 36. First end terminals of potentiometers35 and 36 are connected to negative potential terminal 23, the secondend terminals of these potentiometers being grounded. The electron flowfrom iilament 26 is regulated by a screen electrode 37 which isconnected to an output terminal of emission regulator 29. The gas sampleto be analyzed is introduced into ionization chamber 30 and is theresubjected to electron bombardment so that positive ions are formed.These positive ions are accelerated through tube 10 toward a collectorplate 38 which is mounted at the end of tube 10 opposite filament 26.

The positive ions produced in chamber 3i) are accelerated towardcollector plate 38 by a iirst accelerating grid 40 which is connected toa potential dividing network 41. Negative potential terminal 23 of powersupply 21 is connected to one end terminal of voltage dividing network41, the second end terminal of this network being grounded. The secondaccelerating grid 42 of tube 10 is connected to one output terminal 47of a radio frequency oscillator 43, the second output terminal .ofoscillator y43 being grounded. The output of oscillator 43 is modulatedby the output signal from an audio frequency oscillator 44. Aninductance coil 45 is connected between grids 40 and 42. A thirdaccelerating grid 46 is connected to potential dividing network 41 at apoint which is maintained at a negative potential of lesser magnitudethan the negative potential applied to grid 40. Grids 40, 42, vand 45thus form the rst set of velocity modulating grids. The spacing betweengrids '40 and 42 is equal to the spacing between grids '42 and 46. Asecond set of corresponding grids 40a, 42a and 46a is positioned withintube 1d in spaced relation with grid 4u, 42 and 46. Grid 40a isconnected to grid 46, grid 42a is connected to oscillator outputterminal 47, and grid 46a is connected to potential dividing network 41at a point which is maintained at a negative potential of lessermagnitude than the negative potential applied to grids 46 and 43a. Athird set of corresponding grids 40]), 42b and 46h is positioned inspaced relation with grids 40a, 42a and 46a. Grid '4Gb is connected togrid 46a, grid 42E) is connected to oscillator output terminal 47, andgrid `4Gb is connected to a point on potential dividing network 41 whichis maintained at a negative potential of lesser magnitude than thenegative potential applied to grids 46a and 4Gb. A fourth set ofcorresponding grids 40e, "42C and '46c is positioned in spaced relationwith grids 461i), 4212 and 46]; in tube 10. Grid 40C is connected togrid 46h, grid 42e is connected to output terminal 47 of oscillator 43,and grid 46c is connected to grids '46h and diie. First and secondgroups of grids 50 and 5lY are positioned between this last set ofaccelerating grids and collector plate 3S. Grids 50 are connected to thecontactor of a potentiometer 52, one end terminal of potentiometer 52being connected to positive potential terminal 22 and the second endterminal of potentiometer 52 being grounded. Gr-ids 51 are connected tonegative potential terminal 23. Collector plate 38 is connected to oneinput terminal of a detector circuit 53, the second input terminal ofwhich is grounded.

The positive ions produced within chamberr30 are accelerated towardcollectorY plate 38 by the negative potential applied to grid 4t?.During one-half cycle of the output signal from oscillator 43, theelectrical field between grids 40 and 42 is of such phase that the ionsentering this eld are accelerated. Ions which enter the iield during aparticular phase of this half cycle receive maximum energy. During thenext half-cycle of the signal from oscillator L43, the field betweengrids 42 and 46 is of such phase that the ions are further accelerated.VThese ions then drift through the held-free space between grids 46 and40a. The masses of the individual ions determine their times of arrivalat grid 40a. The ions which arrive at grid 40a at the proper time areagain accelerated by the eld applied between grids 40a and l42a andreceive additional energy. The same accelerating procedure continues asthe ions pass through the next seven grids. Thus, the resulting ion beamis Velocity modulated so that ions of a particular mass receive maximumenergy. The positive potential applied to grids 50 is adjusted such thatonly those ions having a velocity greater than predetermined value areable to pass through grids 50 to impinge upon collector plate 3S. Thepur pose of negative grids 51 is to suppress any electrons which may beformed within tube 10 by the ions bombarding plate'38 or other elementsof the tube. The ions imp inging upon collector plate 3S cause a currentto flow through detector circuit 53 to ground, which current isproportional in magnitude to the number of ions impinging upon collectorplate 33. The current'tlow is amplitude modulated at the same frequencyas the frequency of audio oscillator 44. lt is this modulated componentof the current that is measured in accordance with the present inventionby detector circuit 53.

Power supply circuit 21 is shown in detail in Figure 2. Voltage source2t) is applied across the end terminals of the primary winding 60 of avoltage regulating transformer 61 through a switch 62. lf desired,switch 62 can be a pressure responsive switch which is connected to apressure gage 62 which communicates with the Ainterior of tube 10. Thus,if the vacuum system should fail for any reason, switch 62 opens whenthe pressure withintube a exceeds a predetermined value. This removes'the operating potentials from filament 26 and the various grids tominimize the explosion danger should tube 10 be filled with gas fromsample lines 13 and 15.

The end terminals of a first secondary winding 63 of transformer 61 areconnected to the respective anodes of a double diode 64, the center tapof transformer winding 63` being grounded. The filament of double diode64 is energized by a second secondary winding 68 on transformer 61. Thecathode of double diode 64 is connected to a first positive outputterminal 65 through a pair of series connected inductors 66 and 67. Thejunction between the cathode of double diode 64 and inductor 66 isconnected to ground through a capacitor 70; the junction betweeninductors 66 and 67 is connected to ground throughha capacitor 71; andthe junction between inductor 67 and potential terminal 65 is connectedto ground through a capacitor 72. A resistor 74 and a pair of voltageregulating tubes 75 and 76 are connected in series relation betweenpositive terminal 65 and ground. The junction between resistor 74 andvoltage regulating tube 75 is connected to a second positive potentialoutput terminal 77. A capacitor 78 is connected in parallel with voltageregulating tube 75, and a 'capacitor 79 is connected in parallel withvoltage regulating tube 76. A resistor 81 andk a second pair of Voltageregulating tubes 82 and 83 are connected in series relation betweenpositive terminal 65 and ground. The junction between resistor 81 andvoltage regulating tube 82 is connected to a third positive potentialoutput terminal 22. The junction between voltage regulating tubes 82 and83 is connected to ground through a pair of series connected resistors85 and 86, the junction between resistors 85 and 06 being connected to afourth positive potential output terminal 87: Y i

The operation of the power supply circuit thus far described should nowbe apparent. The rectified voltage provided by double diode 64 isfiltered by inductors 66 and 67 and capacitors "70, 71 and 72. Thevoltage dividing networks formed by the several voltage regulating tubesand resistors 74 and 81 provide output voltages of predeterminedmagnitudes to operate the circuits described in Adetail hereinafter.

In the lower portion of Figure 2 there is shown a second voltageregulating transformer 90 which has a primary winding 91 connected inparallel with transformer winding 60. The end terminals of the rstsecondary winding 92 of transformer 90 are connected to the respectiveanodes of a double diode 93. The filament of double diode 93 isenergized yby a second secondary winding 96 of transformer 90. Thecathode of double diode 93 is Vconnected to the anode and to the screengrid of a pentode 94 through an inductor 95. A capacitor 97 is connectedbetween the center tap of transformer winding 92 and the junctionbetween inductor-95 and the cathode of double diode 93. A secondcapacitor 98 is connected between the center tap of transformer winding92 and the anode of pentode 94. The supressor grid of pentode v94 isconnected to the cathode thereof, and the cathode is connected toground. The center tap of transformer winding 92 is connected to anoutput terminal 23 which is maintained at a constant negative potential.

A voltageregulating tube 101 and a resistor 102 are connected in seriesrelation between terminal 23 and ground. The junction between voltageregulating tube 101 and resistor 102 is connected to the control grid ofa triode 103 through series connected resistors 104 and 105. Athermistor 107, having a negative coeiiicient of thermal resistivity, isconnected in parallel with resistor 104. The junction between resistors104 and 105 is connected to terminal 23 through a resistor 108. Acapacitor 109 is connected in parallel with resistor 108. The cathode oftriode 103 is connected to terminal 23 throughy a -resistor 110 and toground through a resistor ,111. The anode of triode y103 is connected toground 6 through a resistor 1.1.2 and directly to the control grid of atriode 113. The cathode of triode 113 is connected to terminal 23through a resistor 115 and to ground through a resistor 116. Theiilament of triode 113 yis connected to the cathode of triode 113through a resistor 117 and to ground through a capacitor 118. Acapacitor 119 is connected between terminal23 and ground. The anode oftriode 113 is connected to ground through a resistor 121 and to thecontrol grid of pentode 94 through a resistor 122. Heating current forthe filaments of triodes 103 and 113 and pentode V94 is supplied by awinding 124 Vof transformer 90 having output terminals ,r

and y.

The electron current flow through double diode 93 passes through pentode94 and filter inductor 95. In this manner, the potential at terminal 23,which is con'- nected to the center tap of transformer winding 92, ismaintained at a constant negative value. Voltage regulating tube 101 andresistor 102 form` a potential dividing network between terminal 23 andground. Voltage regulating tube 101 is in turn shunted by a potentialdividing network comprising resistors 104 and 108. Thermistor 107 isconnected in parallel with resistorr 104 to compensate for any ambienttemperature changes which may affect the voltage regulation of tube 101.It has been found that an increase in ambient temperature causes thevoltage across the tube 101 to decrease. For example, a change fromabout 25 C. to 60 C. causes a decrease of about 0.2 volt across tube101. If the ambient temperature of tube 101 increases, the voltageacross the tube decreases and this in turn tends to make the voltageapplied to the control grid of triode 103 become more negative. However,this same temperature increase causes the resistance of thermistor 107to decrease so that the ratio of voltages across network 107, 104 and108 is changed. This in turn tends to make the voltage applied to thecontrol grid of triode 103 less negative so that the temperature changeshave no effect on the voltage regulating action of the circuit includingtube 101. if the temperature should decrease, the opposite regulationtakes place. Tube 101 and thermistor 1 07 are maintained in thermalcontact.

The normal voltage regulating action of this circuit can be explained byassuming that the potential at terminal 23 becomes more negative. Thisin turn makes the potential applied to thecathode of triode 103 morepositive with respect to the control grid so that less current flowsthrough triode .103 and anode resistor 112. This makes the potential onthe anode of triode 103 more positive, which potential is also appliedto the control grid of triode 113 to increase the current flow throughtriode 113 and through anode resistor 121. The potential at the anode oftriode 113 is thereby made more negative. This more negative potentialis applied to the control grid of tube 94 to increase voltage dropacross tube 94, and this in turn makes the potential at terminal 2,3more positive. The circuit thus operates to amplify voltage iiuctuationat terminal 23 to restore the potential to the desired value. If thepotential at terminal 23 should tend to become more positive, theabove-mentioned potential changes are reversed so that the voltage dropacross tube 94 decreases to restore the potential at terminal 23 to thedesired value.

As previously mentioned, it is important to direct an electron beam ofconstant magnitude into the ionization chamber ofthe mass spectrometertube in order that the number of ions formed is a function only of thegas pressure in the tube. The electron emission regulator circuit ofFigure 3 is provided for this purpose. Filament 26 is heated by thevoltage applied between terminals 24 and 25 from transformer Winding 89of Figure 2. The contactor of potentiometer 27 is connected to thejunction between a resistor and the anode of a voltage regulating tube131. The second terminal of resistor 130 is grounded. The cathode oftube 131 is connected lto 7 thecontrol grid of a pentode 132 and to oneend terminal of a resistor 133, the second end terminal'of which isconnected to a variable resistor 134. The contactor of variable resistor134 is connected to negative potential terminal 23. The anode of pentode132 is connected to ground through a resistor 135 and directly to thescreen electrode 37 in tube 10. The cathode of pentode 132 is connectedto terminal 23 through a resistor 136 and to ground through a resistor137. The suppressor grid of pentode 132 is connected to the cathode ofpentode 132, and the screen grid of pentode 132 is connected to ground.

The electron current emitted from heated filament 26 is supplied fromnegative terminal 23 through resistors 134 and 133 and voltageregulating tube 131 to the contactor of potentiometer 27. If theelectron emission from filament 26 should tend to increase, thepotential applied to the control grid of pentode 132 tends to becomemore positive because of the increased current ow through resistors 133and 134. This change in control grid potential increases the currentiiow through pentode 132 and anode resistor 135. The increased currentflow through resistor 135 makes the potential at the anode of pentode132 more negative and the potential applied to screen electrode 37 morenegative. The change in po- -tential on screen electrode 37 tends toimpede the electron ilow therethrough such that the electron flow intoionization chamber 30 tends to decrease by an amount suicient tocompensate for the increased current emission from lilament 26. If theelectron emission from iilament 26 should tend to decrease, theabove-mentioned potential changes are reversed such that the potentialapplied to screen elect-rode 37 is made more positive. This in turntends to increase the electron diow into 'ionization chamber 30. By theuse of the emission regulator of Figure 3 it has been found that theelectron flow into ionization chamber 30 can be maintained constantirrespective of small yfluctuations in power supplied to filament 26 andchanges in the emitting properties of filament 26.

As previously described, the potential applied to grids 42, 42a, 42b and42e is the sum of a D.C. voltage and a radio frequency voltage which ismodulated by a frequency in the audio range. The circuit illustrated inFigure 4 is adapted to provide this modulated radio frequency potential.The audio frequency signal is supplied by oscillator 44 which has itsfrequency established by -a vibrating tuning fork 150 which isconstructed of magnetic material. A irst inductance coil 151 is mountedadjacent one arm of tuning fork 150 on a permanent magnet 148, and asecond inductance coil 152 is mounted adjacent the second arm of tuningfork 150 on a permanent magnet 149. One end terminal of inductance coil151 is connected to the control grid of a triode 153, the second endterminal of inductance coil 151 being grounded, as is tuning fork 150.One end terminal of inductance coil 152 is connected to the cathode of asecond triode 154, the second end terminal of inductance coil 152 beinggrounded. The anode of triode 153 is connected to positive potentialterminal 77 through series connected resistors S, 156 and 157 and to thecontrol grid of triode 154 through a capacitor 153 and a resistor 161connected in series relation. The cathode of triode 153 is connected toground through a resistor 159. The control grid of triode 154 isconnected to ground through series connected resistors 161 and 160. Theanode of triode 154 is connected to terminal 77 through series connectedresistors 162, 156 and 157. The anode of triode 154 is also connecteddirectly to the control grid of a triode 163. The anode of triode 163 isconnected to terminal 77 through resistor 157, and

the cathode of triode 163 is connected to ground through a resistor 164.The cathode of triode 163 is also connected to the control grid of atriode 165 through a capacitor 166. 'The control grid of triode 165 isconnected to ground through a resistor 167. The anode of triode isconnected to terminal 77 throughseries connected resistors 169 and 157and to the suppressor grid of a pentode 170 through a capacitor 171. Thecathode of triode 155 is connected to ground Ithrough a resistor 172which is shunted by a capacitor 173. A capacitor 175 is connectedbetween ground and the junction between resistors 157 and 169, and acapacitor 168 is connected between ground and the junction betweenresistors 155 and 156. Heater current for the filaments of triodes 153,157, 163 and 165 is supplied by terminals x and y of the power supplycircuit of Figure 2.

The oscillator circuit thus far described is adapted to provide anoutput signal of frequency in the audio range. This output signal is ofthe same frequency as the frequency of vibration of tuning fork 150.Energy is supplied to tuning fork 150 by coil 152 and magnet 149. Theoutput signal from triode 154 is further amplified by triode 163connected as a cathode follower to isolate the oscillator from thefollowing amplification stage. The output signal from triode 163 isamplified by triode 165 that is biased to operate as an overdrivenamplifier whereby its output signal, which is applied to the suppressorgrid of pentode 170, has substantially a square Wave form. Y

Pentode 170 and the circuit components associated therewith function toprovide oscillations of a radio frequency. These oscillations aremodulated Yby the audio frequency signal applied to the supressor gridof pentode 170. The control grid of pentode 170 is connected t0 groundthrough a crystal which regulates the frequency of oscillations. Crystal180 is shunted by an inductor 131. The cathode of pentode 170 isconnected to ground through a resistor 182 which is shunted byV acapacitor 183. The suppressor grid of pentode 170 is connected to groundthrough a resistor `184. The anode of pentode 170 is connected to apositive terminal 185 through a tuned circuit 186 which comprisesaninductor 187 and a capacitor 188 connected in parallel relation.Terminal is connected to ground Ithrough a capacitor 191. The screengrid of pentode 170 is connected to one terminal of a second tunedcircuit 192 which comprises an inductor 193 and a capacitor 194connected in parallel relation. The second terminal of tuned circuit 194is connected to terminal 185 through a voltage regulating tube 195 whichis shunted by a capacitor 196. The junction between the second terminalof tuned circuit 192 and voltage regulating tube 195 is connected toground through a resistor- 197 that is shunted by a capacitor 198. Theoutput signal from radio frequency oscillator 43 is removed from aterminal 189 which is connected to a point on inductor 187 through acapacitor 190. The output signal which appears between terminal 189 andground thus constitutes a radio frequency voltage, the envelope of whichrepresents the square wave output of audio oscillator 44.

For satisfactory operation of the mass spectrometer, it is importantthat the output signal from oscillator 43 be maintained at a constantvalue. This is accomplished by a voltage regulating circuit. The anodeof pentode 170 is connected through a capacitor 202 lto the anode of afirst diode 203 and to the cathode of a second diode 204.- The cathodeof diode 203 is connected to one end terminal of a potentiometer 205.`The 4second end terminal of potentiometer 205 is connected to the firstend terminal of a resistor 206, the second end terminal of resistor 206being grounded. A pair of parallel connected capacitors 207 and 2018 isconnected between the cathode of diode 203 and ground. The anode ofdiode 204 is connected to ground. The contactor of potentiometer 205 isconnected to the control grid of a pentode 210. The cathode of pentode210 is connected to ground through a voltage regulating tube 211, andthe anode of pentode 210 is connected to positive potential terminal 65through a resistor 212. A cathode of pentode 210 is also connected toterminal 185 through a resistor 213. The suppressor grid of pentode 210is connected to the cathode thereof, and this cathode is connected toground through a capacitor 215. A capacitor 216 is connected betweenterminal 185 and ground. The anode of pentode 210 is connected through aresistor 217 to the control grids of a pair of triodes 218 and 219. Thecathodes of triodes 218 and 219 are connected to one another and toterminal 185. The anodes of triodes 218 and 219 are connected to oneanother and to terminal 65. Y

rl`he output signal from pentode 170 is rectified by the voltagedoubling rectier circuit comprising diodes 203 and 204 and capacitors202, 207 and 20:3. The rectified voltage is in turn applied across thepotential dividing network comprising potentiometer 205 and resistor206. If thisV rectified voltage should tend to increase, the potentialapplied to the control grid of pentode 21d is increased such that thecurrent ow through pentode 210 and anode resistor 212 is increased,which decreases the potential applied to the control grids or" triodes218 and 219. This decreases the current ow through triodes 21S and 219such that the potential at terminal 185 also is decreased. This decreaseof potential `is in turn applied through tuned circuit 186 to the anodeof pentode 170 and through tuned circuit 192 to the screen grid ofpentode 170 such that the output signal from pentode 170 is decreased inmagnitude by an amount sucient to restore the amplitude of output signalto its original value. Conversely, if the rectified voltage appliedacross potentiometer 205 `and resistor 206 should tend to decrease, theabove-mentioned potential changes are reversed so that the voltage-applied to the anode and screen grid of pentode 170 is increased. Inthis manner the ioutput signal of oscillator 43 is maintained at aconstant value.

Mass spectrometer 10 is operated to limit the passage of ionstherethrough to those ions having a predetermined mass. These selectedions impinge collector plate 38 and provide a current flow in detectorcircuit 53, the magnitude of which is a function of the ions impingingplate 38. In order to simplify the detection of this current, the radiofrequency accelerating potential is modulated by audio oscillator 44 sothat the output signal from plate 38 is also modulated at the frequencyof oscillator 44. The magnitude of the components of the output signal`of the frequency of oscillator 44 is measured in accordance with thisinvention to determine the magnitude of the ion beam impinging collectorplate 38. The circuit of Figure 5 comprises a tuned amplifier whichforms the first stage of detector circuit 53. This amplier is tunedtopass signals of the frequency of oscillator 44.

Collector plate 38 is connected to the control grid. of a pentode 225.This control grid is connected to ground through a tuned circuit 226which comprises a capacitor 227 connected in parallel with a seriesconnected inductor 228 and thermistor 229. The cathode of pentode 225 isconnected to an input feedback terminal 230 through a resistor 232i, andthe anode of pentode 225 is connected to positive terminal 65 through aresistor 232. The anode of pentode 225 is also connected to the controlgrid of a pentode 234 through a capacitor 235. The screenv grid ofpentode 225 is connected to terminal 65 through a resistor 236 and toground through a capacitor 237. The suppressor grid of pentode 225 isconnected to the cathode thereof. The anode of pentode 234 is connectedto terminal 65 through a resistor 23S,- and the ycathode of pentode 234is connected to ground through a resistor 240 which is shunted by acapacitor 241. The control grid of pentode 234 is Aconnected to groundthrough a resistor 242, and the suppressor grid of pentode 234 isconnected to the cathode thereof. The anode of pentode 234 is connectedto the control grid of a triode 2,45 through a resistor 246. The cathodeof triode 245 is connected tol ground through aresistor 247 and apotentiometer 248 which are connected in series relation. The contactorof potentiometer 248 is connected to the screen grid of pentode 234. Theanode of triode 245 is corrnected to the anode of a second triode 250,and these two anodes are connected to terminal 65n through apre'- sistor251 and to ground through a capacitor 252. The cathode of triode 245 isconnected to the control grid of triode 250 through a capacitor 254 anda resistor 255 connected in series relation. The cathode of triode 245is also connected to an output terminal 256 through a resistor 257. Thecathode of triode 250 is connected to a feedback output terminal 260through a resistor 261 which is shunted by a capacitor 262. The cont-rolgrid of triode 250 is connected to ground through a resistor 263. Feed'-back terminal 260 is connected to one end terminal of a potentiometer265. The second end terminal of potentionieter 265 -is connected toground, and the contacter of potentiometer 265 is connected to thecathode of pentode 225 through feedback terminal 230.

The tuned ampliier circuit thus far described provides an amplifiedsignal between terminal 256 and ground which is representative of thealternating component of the ion current which is of the same frequencyas the frequency of oscillator 44', circuit 226 being tuned to thisfrequency. The gain of this amplifier is adjusted by feedbackpotentiometer 265 and potentiometer 24d. The output signal appearingbetween terminal 256 and ground is further amplied lby arsecond tunedampliiier, also illustrated in Figure 5, which is generally similar tothe ampliiier above-described and wherein corresponding components aredesignated by like primed reference numerals. A feedback network, whichis illustratedl in Figure 6; is connected between terminals 260 and 230in place of a potentiometer corresponding to potentiometer 265. Acapacitor 263 is connected inA parallel with resistor 231.

The feedback network connected between terminals 260 and 230 isillustrated in Figure 6. Terminal 260' is connected to one end terminalof a potentiometer 270, the second end terminal of potentiometer 270'being oonnccted to the arm 271 of a relay 272. t vitch arm 271 normallyengages a contact 273 which is connected to one end terminal of a.potentiometer 274. The second end terminal of potentiometer 274 isconnected to' ground. A capacitor 283 is connected between the endterminals of potentiometer 270. When current is passed to the coil ofrelay 272, switch arm 271 engages a second contact 276 which isconnected toy one end terminal of a potentiometer 277. The second endterminal of potentiometer 277 also' is connected to ground. Thecontacter of potentiometer 274' is connected to a terminal 279 which isengaged by a switch arm 280. Switch arm 2801s connected to terminal 230.In its second position switch arm 280 l engages a contact 281 which isconnected to the com tactor of potentiometer 277 The contactor ofpotentiometer 270 is connected through` a capacitor 284 to the anode ofa diode 235. Thev anode `of diode 285 isl connected to ground through aresistor 423. The cathode of diode' 285 is connected to ground through aresistor 286 which is shunted by a capacitor 287. The cathode of diode285 is also connected to one input terminal 288 of a servo amplifier290. The second input terminal 291 of amplifier 290 is connected to thecathode of a second diodo 292. The cathode of diode 292 is connected toground through a resistor 293, and the anodevof diode 292 is connectedto positive potential terminal 77 through` a resistor 295 and to groundthrough a resistor 424. One output terminal 297 of amplifrer 290 isconnected to ground and the second output .terminal 298 is connected tothe switch arm 3550 of a relay 301. In the absence of current beingapplied to the coil of relay 301, switch arm 300' engages a terminal 302which is connected to one terminal `of the first coil 303 of areversible two-phase induction servo motor 304. The second terminal ofcoil 303 is connected to ground. When current is applied to the coil ofrelay 301, switch arm 300 engages a terminal 305 which is connected toone terminal of the first coil 306 of a second reversible two-phaseinduction servo motor 307. The second terminal of coil 306 is connectedto ground. The second coils 309 and 310 `of respective -motors 304 and307 are connected -across voltage terminals 311 and 312 which aredescribed in conjunction with Figure 7.

In normal operation of the feedback circuit of Figure 6, relays 272 and301 are not energized such that switch arm 271 engages terminal 273 andswitch arm 300 engages terminal 302. The voltage appearing between thecontactor of potentiometer 270 and ground is rectified by diode 285 andapplied to input terminal 288 of servo amplifier 290. This voltage is inturn compared with a constant reference voltage which is applied toinput terminal 291. Any difference between the voltages applied to inputterminals 288 and 291 results in an output signal from amplifier 290Which drives motor 304 to adjust the position of the contactor ofpotentiometer 274, the contactor of potentiometer 274 being mechanicallycoupled to the drive shaft of motor 304. The contactor of potentiometer274 is moved in a direction to eliminate the potential differencebetween terminals 288 and 291 ot amplifier 290. It can be seen thattheposition of the contactor of potentiometer 274 regulates the feedbacksignal applied between terminals 260 and 230 of the amplier of Figure 5.This varies the gain of the second section of the tuned amplifier ofFigure 5. The detector circuit is thus operated so that the gain of thetuned amplifier is varied whereby the output signal therefrom is of aconstant magnitude. The degree of rotation of servo motor 304 isnecessary to restore the tuned amplifier circuit to this condition ofbalance is a function of the magnitude of the ion current impinging uponcollector plate 38. The rotation of motor 304 can be observed by acalibrated dial, not shown, associated with potentiometer 274 and by atelemetering transmitter which comprises a potentiometer 315 having oneend terminal thereof connected to potential terminal 87 and the secondend terminal conl nected to ground. The output terminals 316 and 317 ofthis telemetering transmitter are connected to the contactor ofpotentiometer 315 and to ground, respectively. The contactor ofpotentiometer 315 is also mechanically coupled to the drive shaft `ofmotor 304.

Servo amplifier 290 is illustrated in detail in Figure 7. Input terminal288 is connected to the junction between iirst end terminals ofrespective primary windings 325 and 326 of a transformer 327. inputterminal 291 is connected to a vibrating reed 328 which is moved by acoil 329 to engage periodically terminalsv 330 and 331 which areconnected to the respective second end terminals of transformer windings325 and 326. The end terminals of coil 329 are connected to therespective end terminals of a secondary winding 333 of a transformer334. The primary winding 335 of transformer 334 is connected across theoutput terminals of voltage source 20. A potentiometer 336, having agrounded center tap, is connected in parallel with coil 329.

One end terminal of the secondary winding 337 of transformer 327 isconnected to the control grid of a 4pentode 338. The second end terminalof transformer .winding 337 is connected to ground. A capacitor 339 isconnected in parallel with transformer winding 337. The cathode ofpentode 338 is connected to ground through a resistor 341 which isshunted by a capacitor 342. The anode of pentode 338 is connected topositive potential terminal 65 through Series connected rc- -sistors 343and 344. A capacitor 345 is connected between ground and the junctionbetween resistors 343 and 344. The suppressor grid of pentode 338 isconnected to the cathode thereof, and the screen grid of pentode 338 isconnected to the contactor of a potentiometer 346. One end terminal ofpotentiometer 346 is connected to terminal 65 through a resistor 347 andthe second end terminal of potentiometer 346 is connected to groundthrough a resistor 348. The anode of pentode 338 is also connected toone end terminal of a potentiometer 350 through a capacitor 351. Thesecond end terminal of potentiometer 350 is connected to ground, and thecontactor of potentiometer 350 is connected to the control grid of apentode 352. The cathode of pentode 352 is connected to ground vthrougha resistor 353, and the anode of pentode 352 is connected to terminal 65through a resistor 354. The anode of pentode 352 is also connectedthrough a capacitor 356 to the control grids of four triodes 357, 358,359 and 360. The screen grid of pentode 352 is connected to terminal 65througha resistor 362 and to ground through a capacitor 363.

The control grids of triodes 357, 358, 359 and 360 are connected toground through a common resistor 364. The cathodes of triodes 357, 358,359 and 360 are connected to one another and to ground through a commonresistor 365. The anodes of triodes 357 and 360 are connected to oneanother and to the rst end terminal of a secondary winding 366 oftransformer 334. The anodes of triodes 358 and 359 are connected to oneanother and to the second end terminal of transformer winding 366. Thecenter tap of transformer Winding 366 is connected to amplier outputterminal 298. -A capacitor 368 is connected between terminal 298 andgrounded terminal 297. `One output terminal of voltage source 20 isconnected to output terminal 311 `and the second output terminal ofvoltage source 20 is connected to terminal 312 through a capacitor 369.

Reed 328 is vibrated between contacts 330 and 331 by coil 329 at thefrequency of voltage source 20, which can be sixty cycles per second,for example. The input signal appearing between terminals 288 and 291 isthus applied alternately to transformer windings 325 and 326 so that asixty cycle signal of magnitude proportional to the signal appearingbetween terminals 288 and 291 is applied to the input of pentodeamplifier 338. The output signal from pentode 338 is in turn applied tothe input of pentode 352 and the output of pentode 352 is applied to theinput of the four triodes 357, 358, 359 and 360. V

The voltage from source 20, which is applied to the anodes of triodes357 and 358 through transformer 334, results in these two anodes beingpositive during alternate half cycles of the applied voltage. In theabsence of a signal being applied to the control grids of triodes .357and 358 from pentode 352, the output of the two triodes 357 and 358consists of two pulses of current per cycle of applied voltage fromsource 20. If these two pulses are of equal magnitude, which theynormally are because of the balanced circuit, there is no sixty cyclecomponent in the output signal which appears between terminals 298 and297. However, if a sixty cycle signal, either in phase with or 180 outof phase with the operating voltage supplied by source V20, is appliedto the control grids of triodes 357 `and 358, one of the output pulsesfrom these tubes is increased and the other decreased to provide a sixtycycle component in the output signal between terminals 298 and 297. Byproviding triodes 359 and 360 in parallel with respective triodes 358and 357, a safety factor is established because if one of the triodesshould fail the other will continue to operate. 297 is applied acrosscoils 303 and 306 of respective motors 304 and 307 depending upon theposition of switch arm 300. The voltage appearing between terminals 311and 312 is applied across coils'309 and 310 of respective motors 304 and307. This latter voltage is out of phase with `the voltage appliedacross the The output signal between terminals 298 and first-mentionedcoils so as to provide a rotating' magnetic iield. The direction ofrotation of motors 304 and 307 depends upon the relative magnitude ofthe potentials applied to terminals 288 and 291 of amplier 290. If the.potential at` terminal 288 exceeds the potentialat terminal 291 themotors rotate in iirst directions, while if. the relative magnitude oithese potentials is reversed the. motors rotate in opposite directions.

r.It will bev noted that the gain of pentode 338 in ampliiier 290 isdetermined in part by the potential applied to the screen grid thereoffrom the voltage dividing circuit comprising resistors 347 and 348 andpotentioemter 346. The contacter of potentiometer 346 is mechanicallycoupled to motor 384 and is adjusted by the output rotation or' thismotor. As the contactor of potentiometer 2.74 (Figure 6) is moved fromone end terminal to the other, the potential applied to the screengridof pentode 338 is simultaneously varied. As the contactor ofpotentiometer 27 4 moves tov/ard the grounded endterrninal to reduce thefeedback signal and increase thev amplier gain, the contactor ofpotentiometer 346 is moved to'reduce the potential applied to the screengrid of pentode 338 to decrease the gain of servo amplier 290'.v Theimportance of this variable gain potentialV dividing network shouldbecome apparent from the following examples. it is rst assumed that theoutput signalY from spectrometer tube 10 is large so that the positionof the contacterV of potentiometer 274 is at its upper limit whichVprovides maximum feedback. and minimum tuned ampliiier gain. itis nextassumed that the output signal from tube liti drops to one-half its ini?tial value. Y r1`he tuned amplifier gain must then be .doubledV tomaintain a constant output. The contactor orf-potentiometer 274 must,accordingly, move to its midpoint to double this gain. Thus, for atWo-to-one gain variation, motor 304 must mover the contactor ofpotentiometerV 274V over fty percent of its total path. it isf nextassumed that the output signal from tube 10 is only one-tenth'A ot'itsfull value and that the contacter of potentiometer 274 is' one-tenth ofthe distance between its; lower a-nd upper end terminals. Under thisconditionV the amplifier gain is ten times the value initially assumed.If the output signal from tube 10 is againhalved, the contact'or ofpotentiometer 274 must be moved to a point one-twentieth of the distancebetween the lower and upperend terminals. For a two-to-one gainvariation under these latter conditions, motor 304 moves-thecontactorof` potentiometer 274 over only ve percent ofits total-path.-

From a comparison of the two examples it can be seen thatthedirectcurrent voltage difference applied to servo amplifier 290 is some tentimes as largev per degree of movement of the contactor of potentiometer274 when thecontactor is at its lower end than at the upper end. Thisrequires the servo ampliiier gain in the lirst situation to beapproximately ten times greater than that required for the secondsituation. However, by varying the gain of pentode 338, the gain ofamplifier 290 can be variedy in a non-linear fashion which nearlymatches the nonlinear change in output signal per unit change of theposition ofy thecontacter of potentiometer 274. The gain of amplifier290 is not critical because this amplifier is employed merely to providesuflicient power to rotate motors 304y and 307m eliminate the voltagediierence between-terminals 288 and 291.

rlimer circuit 16 is illustrated in Figure'. The purpose of this timeris to pass a standard gas sample into ionization chamber 38 periodicallyin place ofthe gas sample under analysis so that the operation of themass spectrometerv tube and circuitry can be checked and adjusted asnecessary to retain the instrument properly calibrated. YThe timer isactuated by a synchronous motor 375 which is connected across voltagesource 20. Motor 375 drives a pair of cams 376 and 377 at a con- 14stant speed Vby suitable gearing, not shown. The coils of relays 272 and301 are energized by a source of direct voltage appearing betweenterminals 378 and 379'. One output terminal of voltage source 20 isconnected to ter"-l minal 378 through a rectifier 380 and the secondoutput terminal of voltage source 20 is connected directly to terminal379. A lter capacitor 381 is connected in parallelV with terminals 378and 379. The coils of relays 272 and 301 are connected across terminals378l and 379 through a switch 383 which is operated by cam 376.Solenoids 13a andy 15a, which operate respective valves 13 and -15(Figure l), are connected across terminals 378 and.k 379 through aswitch 384 which is operated by cam 377. During a predetermined portionof one cycle of rotation of cams 376 and 377, switches 385 and384 remainopen. However, during a second predetermined portion oi one cycle ofrotation of cams 376 and 377, switch 383 is closed so that relays 272and 301' are energized. This moves switch arms 271 and 300 intoengagement with respective contacts 276 and 305. Relay 301 thusterminates the rotation of motor 304 and. energizes motor 307 in placethereof from the output signal of amplifier 290. Potentiometer 277 islconnected in circuit with potentiometer 278 in place of potentiometer274. by relay 272. At the same time,.solenoids 13a and 15a are energizedby. closure of switch 384 to close valveV 13 and open valve 15. Thisallows areference gas to pass into ionization chamber 30.

If no change in the operation of the electrical circuitry associatedwith the spectrometer tube or the tube itself has. occurred since theprevious standardization period, the voltage applied to. servo amplifier290 remains zero so that motor 307 remains stationary. However, if` anydrifthasoccurred inthe system the input voltage applied toamplilier290results in rotation of motor 307 to vary the position` of thecontacter of the potentiometer 270, which is mechanically coupledto the'drivev shaft` of motor 307, by an amount suiicient to correct for anydriftv in the circuitry.

The overall operation oi the mass spectrometer ofthis invention shownnow become apparent. With reference to- Figure l, electrons emitted fromheated filament 26 are accelerated into ionization chamber 30. Theelectron lioW into ionization chamber 30'is maintainedconstant byemission regulator 29. The positive ions formed in chamber 30 areaccelerated toward collector plate 38-by the-negative potentialsVapplied to grids 33 and 34. Ions'.- of alparticular mass are furtheraccelerated by means of the modulatedV radio frequency potential appliedto gridsv 42 42u, 42b and 42e. The ionsv which acquire suficient energyyto overcome the positive potential barriermaintained at grids.VS'impinge collector plate 38 to actuate detector 53.

'Iheispacings s between individual grids ofeach'set are maintainedequal, whereas the'spacings rbetween th'esets of grids. arerepresented'by the experssion.:

where? n2 is'A an integralV number and tV is' the thickness of eachgrid, `all dimensions being in inches. In one embodiment of thisinvention, the following spacings were utilized: grid 42 was 0.118 inchfrom grid 40,' grid' 46 was 0.118 inch from grid 42, grid 40a was 2.618inches fromgrid 46, grid 42a was 0.118 inch from grid 40a, grid' 46awas0.118 inch fromv grid 42a, grid 40b was 1.345 inches from grid 4de, grid42h was 0.118finch froru grid` 40b, grid 46b was 0.118 inch from grid42h, grid 40e was 1.982 inches from grid 4Gb, grid 42C was 0.118 inchfrom grid 40C, and grid 45C was 0.118 inch from grid 42C. Inthisarrangement s is 0.118, t is 0.01, the n between grids 46 and 40a isnine, the n between grids 46a and 40b is five, and the n betweenv grids`4Gb and 40e is seven. Other values of n can be employed if desired,andi the number of drift spaces can be varied if desired.

15 In this same embodiment of the invention the circuit components ofpower supply 21 were as follows: resistor 74, 4000 ohms; resistor 81,33,000 ohms; resistor 85, 75,000 ohms; resistor 86, ohms; resistor 122,1000 ohms; resistor 121, 560,000 ohms; resistor 112, 500,000 ohms;resistor 116, 15,000 ohms; resistor 117, 2.7 megohms; resistor 115,15,000 ohms; resistor 110, 10,000 ohms; resistor 111, 30,000 ohms;resistor 105, 100,000 ohms; resistor 108, 1 megohm; resistor 104, 8,200ohms; resistor 102, 120,000 ohms; capacitor 70, 4 microfarads; capacitor71, 40 microfarads; capacitor 72, 60 microfarads; capacitors 78 and 79,0.02 microfarad each; capacitor 97, 30 microfarads; capacitor 98, 40microfarads; capacitor 118, 0.1 microfarad; capacitor 119, 30microfarads; capacitor 109, 0.1 microfarad; inductors 66, 67 and 95, 13henries each; diodes 64 `and 93, type 5V4; tubes 75 and 76, type CA2;tubes 82, 83 and 101, type 5651; pentode 94, type 6Y6; and triodes 113and 103, type 6SL7. The voltage supplied by source 20 was 115 volts, 60cycles. The voltage between terminals 24 and 25 was approximately 6.3volts. The voltages at the output terminals were: 65, 370 volts; 77, 300volts; 22, 174 volts; 87, 10 millivolts; and 23, negative 330 volts.

In Figure 3, potentiometer 27 had `a total resistance of 1000 ohms;resistor 130, 50,000 ohms; resistor 135, 560,- 000 ohms; resistor 137,20,000 ohms; resistor 133, 50,- 000 ohms; potentiometer 134, 50,000ohms; resistor 136, 20,000 ohms; pentode 132, type 6AU6; and tube 131,type 5651.

In Figure 4, tuning fork 150 vi'brated at 1000 cycles per second;resistors 155 and 162 were 300,000 ohms each; resistors 157, 160, 167and 206, one megohm each; resistor 159, 1200 ohms; resistor 161, 300,000ohms; resistor 156, 220,000 ohms; resistor 164, 100,000 ohms; resistor169, 10,000 ohms; rresistor 172, 6,800 ohms; resistor 157, 10,000 ohms;resistor 184, 100,000 ohms; re-

. sistor 182, 180 ohms; resistor 212, 560,000 ohms; re-

sistor 214, 820,000 ohms; resistor 217, 470 ohms; resistor 213, 50,000ohms; resistor 197, 18,000 ohms; capacitor 158, 0.01 microfarad;capacitor 168, 0.1 microarad; capacitor 166, 0.1 microfarad; capacitor173, 0.001 microffarad; capacitor 175, 4 microfarads; capacitor 171, 0.1microfarads; capacitor 188, 200 micro-microfarads; capacitor 191, 0.002microfarads; capacitor 194, 300 micro-microarads; capacitor 183, 0.002microfarad; capacitor 202, 0.0015 microfarad; capacitor 190, 0.0015microfarad; capacitor 207, 0.0015 microfarad; capacitor 208, lmicrofarad; capacitor 198, 0.002 microfarad; capacitor 215, 0.02microfarad; capacitor 216, 20 microfarads; inductor 81, 2.5millihenries; circuits 186 and 192, each tuned to 3.7 megacycles;triodes 153, 154 and 163, 165, type 12AX7; pentode 170, type 6AS6;diodes 203, 204, type 6AL5; pentode 210, type 6AU6; triodes 218, 219,type 12AU7, tube 195, type CB2; and tube 211, type 5651. Crystal 180 hada resonant frequency of 3.7 megacycles. be varied from approximately 175to approximately 205 volts.

With reference to Figure 5, resistors 232 and 232' were 510,000 ohmseach; resistors 236 and 236', 2.7 megohms each; resistors 238 and 238',15 megohms each; resistors 242 and 242', 1 megohm each; resistors 263and 263', l megohm each; resistors 231 and'231', 2,200 ohms each;resistors 246 and 246', 560 ohms each; resistors 240 and 240', 51,000ohms each; resistorsv255 and 255', 560 ohms each; resistors 251 and251', 10,000 ohms each; resistors 247 and 247', 27,000 Ohms each;potentiometers 248 and 248', 25,000 ohms each; potentiometer 265, 50ohms; capacitors 237 and 237', 0.03 microfarad each; capacitors 268 and268', 10 microfarads each; capacitors 235 and 235', 0.002 microfaradeach; capacitors 241 and 241', 50 microfarads each; capacitors 254 and254', 1.0 microfarad each; capacitors 262 and 262', 0.002 microfaradeach; capacitors 252 and 252', 4 microfarads each; capacitor 227, 0.001micro- The voltage at terminal 185 could farad; capacitor 227', 0.05microfarad; capacitor 239, 10 microfarads; inductor 228, 25 henries;inductor 228', 500 millihenries; potentiometer 265, 50 ohms; pentodes225 and 225', type 5879; pentodes 234 and 234', type 6AU6; and triodes245, 250 and 245', 250', type 5687.

In Figure 6, resistor 295 was 440,000 ohms; resistor 293, 100,000 ohms;resistor 286, 100,000 ohms; resistors 423 and 424, 10,000 ohms each;capacitor 287, 20 microfarads; capacitor 284, 0.22 microfarad; capacitor283, 0.002 microfarad; potentiometer 270, 1,000 ohms; potentiometers274, 277 and .315, 50 ohms each; and diodes 285, 292, type 6AL5.

1n Figure 7, resistor 343 was 510,000 ohms; resistor 344, 100,000 ohms;resistor 354, 470,000 ohms; resistor 362, 1 megohm; resistor 364, lmegohm; resistor 365', ohms; potentiometer 336, 1,000 ohms; resistor353, 1,800 ohms; resistor 341, 2,200 ohms; resistor 347, 180,- 000 ohms;resistor 348, 5,100 ohms; potentiometer 346, 10,000 ohms; capacitor 339,0.0033 microfarad; capacitor 345, 1 0 microfarads; capacitor 351, 0.05microfarad; capacitor 356, 0.05 microfarad; capacitor 342, 50microfarads; capacitor 363, 0.1 microfarad; pentode 338, type 5879;pentode 352, type 6AU6; and triodes 357, 358 and 359-, 360, type 12AU7.

In the operation of the mass spectrometer tube of this invention it isimportant that a tixed relationship between the radio frequency voltageand the negative accelerating potentials be maintained. In this regardit has been found advantageous to modulate the radio frequency signal bya square -wave audio frequency signal. 1f the radio frequency signalwere sine wave modulated, then the radio frequency signal envelope wouldvarypin sinusoidal fashion. Under such circumstances, the radiofrequency voltage for which the tube -is designed to operate lat wouldbe realized for only a very sho-rt interval at the peak of the envelope.The use of square wave modulation, however, permits the radio frequencyenvelope to remain at the proper level for substantially onehalf of themodulating period. This permits an output signal of greater magnitudefromthe collector plate than could be obtained with sine Wave modulationof the radio frequency voltage. The amplitude of the modulating voltageappears to be less critical for `a square wave than -for a sine Wave.

The collector current, which `depends primarily upon the number ofpositive ions of preselected mass that have impinged collector plate 38,is returned to ground through the parallel resonance circuit 226 ofFigure 5. This circuit is tuned to the modulating frequency of 1000cycles per second, for example. The input voltage applied to the'amplier of Figure 5 is, therefore, the product of the collector platecurrent and the impedance of circuit 226. Obviously, the higher theinput impedance the -higher is the voltage applied to the control gridof pentode 225. The resonant impedance of circuit 226 can be expressedas ZzwLQ Where w=2vrf, f is the resonant frequency. L is the inductanceof coil 228,

and R. is the series resistance of coil 228. In the particular circuitQ=40, L=25 henries, f=1000 cycles per second and Z=6.28 10s ohms. Theactual Q of coil 228 is approximately 100. However, to compensate forimpedance variations resulting from ambient temperature variations ithas been `found desirable to include thermistor 229 in series with coil228. This, of course, increases the effective resistance of coil 228 andlowers the Q to the indicated value. The use of such a tuned circuit inthe amplifier input network provides at least two decided advantages. Atuned circuit restricts the band Width of the system and therebyimproves the signal-to-noise ratio. Also, the stray shunt capacitance ofcollector plate 38, the input capacitance of pentode 225 and thecapacitance of the cable connecting plate 38 to the control grid ofpentode 225 are utilized as part of the tuning capacitance in circuit226. If an input resistor were employed in conjunction with pentode 225,this stray capacitance would shunt the input resistance and lower theeffective input impedance which could be obtained. For example, if thetotal shunt capacitance were 50 microfarads, then the reactance at 1000cycles per second is approximately three megohms. Thus, the highestpossible input impedance is approximately three megohms as compared withthe considerably higher input impedance resulting from circuit 226.

Voltage dividing network 41, which is illustrated in Figure l, isemployed to maintain the proper negative potentials on the grids of tube10. Negative potential terminal 23 of power supply circuit 21 is appliedto one end terminal of a first resistor 400. The second end terminal ofresistor 400 is connected directly to a switch terminal 401 and toswitch terminals 402'and 403 through respective resistors 404 and 405. Aswitch arrn 407, which selectively engages terminals 401, 402 and 403,is connected to the end terminal of a variable resistor 408. Thecontactor of variable resistor 408 is connected to grid 40 and to firstend terminals of resistors 409 and 410. The second end terminal ofresistor 409 is connected to the contactor of a variable resistor 411.The end terminal of resistor 411 is connected to the first end terminalof a potentiometer 412 and to the first end terminal of a resistor 413.The second end terminals of potentiometer 412 and resistor 413 vareconnected to one another. The contactor of potentiometer 412 isconnected to the end terminal of a variable resistor 414. The contactorof resistor 414 is connected directly to a switch terminal 403a andtolswitch terminals 402a'and 401a through respective resistors 404aand405a. A switch arm 407a, which is connected to ground, ismechanically coupled to switch'arm 407 and selectively engages terminals401g, 40261 and 403a at the same timeV switch arm 407 engagesrespectively terminals 401,402 and '403. The second end terminalofresistor 410 is connected to the iirst'end terminal of a resistor 416,and the second end terminal of resistor 416 is connected to vthe firstend terminal of a resistor 417. The second end terminal of resistor 417is connected to the junction between variable resistor 411 andpotentiometer 412. This latter junction is also connected to grids 46c,40C and 46b. The Vjunction between resistors 410 and 416 is connected t`grids 46 and 40a, and the junction between resistors 416 and 417 isconnected to grids 46a and 40b.

In the previously mentioned embodiment of the mass spectrometer, thecircuit components of voltage dividing network 41 .were as follows:resistors 410, 416 and 417, 120,000 ohms each; resistors 404 and 404a,8,000 ohms each; resistors 405 and 40511, 16,000 ohms each;potentiometers 408 and 414, 10,000 ohms each; resistor 40.9, v'2,000ohms; potentiometers 411 and 412, 5,000 ohms each; and resistor 413,360,000 ohms. f

Network 41 is designed such that changes can be made in the potentialsapplied to the various grids. 'I'he potential applied to grid 40 isreferred to as the accelerating potential, whereas the potentialsapplied to grids 46, 40a, 46a, 40b, 46h, 40C and 46c are referred to asstep-back potentials. These step-back potentials suiiciently retard.acceleration of the ions sothat selected ions retain proper velocitiesto receive maximum energy from each radio frev q'uency acceleratingfield through which they pass. The

accelerating potential can be varied by either ganged 412.y A motor 420is provided to adjust the accelerating Vpotentials automatically .toscan a sample for the presence ,p of ions,A of various masses.- Motor420 adjusts resistors Y4,08 and v414 to vary the acceleratingy potentialover approximately one hundred volts. Additional variance is -pbtainedby manual adjustment of switches 407 and 407a.

Motor 420 can drive the recorder chart associated with detector 53 suchthat the output signal is correlatedwith the accelerating potentials.

From the foregoing description, it should be apparent that there isprovided in accordance with this invention an improved massspectrometer. This instrument is particularly useful for processanalysis and control because of its small size. The spectrometer doesnot require a magnetic deiiecting field, and as such is less bulky andless expensive to operate than conventional mass spectrometers. Whilethis invention has been described in conjunction with a presentpreferred embodiment, it should be evident that the invention is notlimited thereto.

What is claimed is: v v

1. A massfspectrometer comprising a gas impermeable envelope enclosingan ion source; a collector plate spaced from said soure; a plurality ofgroups of grids spaced between said ion source and said collector plate,each of said groups comprising three equally spaced grids, the spacingsbetween adjacent groups being n-0.3183-(2s|t) inches, where n is anintegral number, s is the spacing between adjacent grids in each group,and t is the thickness of each grid, all of said dimensions being ininches; first and second spaced grids positioned between said collectorplate and the group of said grids adjacent'said collector plate; a firstsource of alternating potential of a first frequency, one terminal ofsaid first source being connected tol the center grid in each of saidgroups, the'second terminal of said lfirst source being connected to apoint of reference potential; a second source of alternating potentialof a second frequency which is lower than said'lirst frequency; means tomodulate said first source by said second source; means to apply apotential of polarity opposite the polarity of the ions being analyzedtosaid first grid; and means to `apply a potential of polarity the same asthe polarity ofthe ions being analyzed to said second-grid..

2l The combination in accordance with claim 1 further comprising meansconnected to said collector plate to measure the magnitude of thecomponent of the l ion 'stream impinging said collector plate of thefrequency of said second frequency.

' 3. A mass spectrometer comprising a gas impermeable -envelopeenclosing an ion source; a collector plate spaced from said ion source;twelve grids spaced consecutively between said ion source and'saidcollector' plate, thefirst `ofsaid grids being adjacent said ion sourceand r the twelfth of Vsaid grids being adjacent said collector plate,

the spacings between said first and second, said second and third, saidfourth and fifth, said fifth and sixth, s aid seventh andY eighth, saideighth and ninth, said tenth and eleventh, andsaid eleventh and twelfthgrids being equal,

veach of said twelve grids having the same thickness, the Vspacingbetween said third and fourth grids vbeing 's is the spacing betweensaid first and second grids and t ytenth grids being n."0.3183(2s|-t)inches, where n" is -an integral number; a thirteenth grid positionedbetween said twelfth grid and said collector plate; means for apply-`ing a potential to said thirteenth grid of polarity opposite 4thepolarity of the ions being analyzed; a source of alternating potential,one terminal of said source being connected to said second, fifth,eighth and eleventh grids, the -second terminal of said source beingconnected to a point ,of reference potential; and means for applyingpotentials of polarity opposite the polarity of the ions being analyzedto said first, third, fourth, sixth, seventh, nineth, tenth and twelfthgrids comprising a potential dividing network, and a source of potentialof polarity opposite the polarity of the ions being analyzed appliedacross said network, said first grid being connected to a first point-on said network which is maintained at a first potential,

said third and fourth grids being connected to a second point kon saidnetwork which is maintained at a second potential of lesser magnitudethan said first potential, said sixth and seventh grids being connectedto a third point on said network which is maintained at a thirdpotential of lower magnitude than said second potential, and said ninth,tenth and twelfth grids being connected to a fourth point .0n saidnetwork which is maintained at a fourth potential of lesser magnitudethan said third potential.

il The combination in accordance with claim 3 further comprising meansto vary the magnitude of said first, second, third and fourth potentialssimultaneously.

5. An ion source comprising, in combination, an ionization chamberincluding inlet means to receive material to be ionized, an electronemitting filament, means for directingelectrons emitted from saidfilament to said ionization chamber, -a screen electrode positionedinthe path Gf' 4Said electrons, a potential dividing network, a voltagesource applied across said network, an electron tube having` at least acathode, an anode and a control grid, an impedance element' having oneterminal connected to said anode, said cathode being connected to oneterminal of said voltage source, the second terminal of said impedanceelement being connected to the second terminal of saidvoltage source,said anode being connected to said vscreen electrode, means connectingsaid filament to a point on said network intermediate the end terminalsthereof, .and means connecting said control grid to a second point onsaid network intermediate the end terminals thereof.

l 6,.; An ion source comprising, in combination, anionpation chamberincluding inlet means to receive material .to be ionized, an electronemitting filament, means Ifo directing electrons emitted from saidfilament to said io ,zativon chamber, a screen electrode positioned inthe path of said electrons, an electron tube having at least a cathode,an anode, and a control grid, a firstV resistor having one terminalconnected to said anode, a voltage sc mrceV .applied between saidcathode Vand the second termittalv of said first resistor, the positiveterminal of said voltage source being connected to said first resistor,a second resistor having one terminal connected to the positive terminalof said; voltage source, a voltage regulat- ,ing4 tube having the anodethereof connectedr to the secterlninal of said second resistor, and athird resistor `having one terminal thereof connectedto the cathode ofsai'dyvoltage regulating tube, the second' terminal of said 'third`resistor being connected to the negative terminal of voltage source, theanode of said voltage regulating Itube being connected? tol said;filament, .and the anode of said-electron tube being connected to said'screen electrode.

7. Circuit means for measuring alternating lcurrent signale comprising,in combination, an alternating current amplifier having a variablefeedback networkincluded therein; to vary` the gain of theampliiiensaid' amplifier comprising an; electron tube having an anode, acathode and a control grid, means to apply a potential between saidanode and saidY cathode, an input terminal connected Ito said controlgrid, a capacitor connected between said control grid and `a point ofreference potential, anfinductorand a resistance element having anegative temperature coefficient of resistivity connected in seriesrelation, said series connected inductor and resistance element beingconnected in parallel with said capacitor;

means to establish a rst voltage of magnitude proportionalto themagnitude of the output signal from said amplifier; a source ofreference voltage; means to compare said first voltage with saidreference vol-tage; means responsive to said comparing means to Yvarysaid feedback network untill ther-eis a zero difference between saidvoltages being compared; and means to measure the magnitude of varianceof said feedback network.

" 8. Means .for measuring alternating current signals comprising, incombination, an alternating current amplifer having a variable feedbacknetworkinclndedtherein to vary the gain of the amplifier, means toestablish a first direct. voltage of magnitude proportional to themagnitude ofthe output signal from said amplifier, a source of referencedirect voltage, means to compare said reference'voltage with said firstvoltage to establish a voltage difference, means to amplify said voltagedifference, a servo motor actuated by said amplified voltage difference,the gain of said servo amplifier being :adjustable, means connectingsaid servo motor to said feedback network so that the gain o-f saidamplifier is adjusted by said servo motor until there is Ia zerodiiference between said voltages being compared, and means under controlof said servo motor to vary the gain of said servo ampliiier so that thegain of said servo vamplifier has a irst value when the amplitude of thesignal being measured has a Ifirst value and has a second value when theamplitude of the signal being measured has a second value.

9. Circuit means for measuring alternating current signals comprising,in combination, an alternating current amplifier having a variablefeedback network included therein to vary the gain `of the amplifier,means to establish a direct voltage of magnitude proportional to themagnitude` of the output signal from said amplifier, a source ofreference direct vol-tage, a converter to establish an alternatingsignal of magnitude proportional to the difference between said directvoltage and said reference voltage, an alternating current servoamplifier to amplify said converted signal, the gain of said servoamplierbeing adjustable, a servo motor connected to said feedbacknetwork, said servo motor being energized by the output ofv said servoamplifier so that said feedback network is adjusted until there is azero diiference between said reference voltage and said direct voltage,and means under control of said servo motor to vary the gain of saidservo amplifier so that the gain of said servo ampliiier has a firstvalue when the amplitude kof the signal being measured has a first valueand has a second value when the amplitude of the signal being measuredhas a second value.

l0. The combination in accordance with claim 1 furtherincluding means tomeasure the magnitude of .the ion stream impinging said collector platecomprising, in combination, means Ito establish a first voltage ,ofmagnitude proportional to lthe magnitude of the output signal' from saidamplifier, a source of reference voltage, means to compare said firstvoltage with said reference voltage, means responsive to said comparingmeans to vary said feedback network until there is a zero rdilferf encebetween said voltages being compared, =and means to measure themagnitude of variance of said feedback network.

11. A constant output oscillator comprising, in combination, an electrontube including an anode, a cathode and -a control grid, a source ofpotential, a tuned circuit comprising an inductor and a capacitorconnected in parallelrelation, a variable resistance element, said anodebeing connected to one terminal of said tuned circuit, the secondterminal of said variable resistance elem-ent being connected to saidsource of potential, circuit means connected` to said control grid tosustain oscillations in said .tuned circuit, and means to vary theresistance of said resistancetelement in response to potential changesat the anode of said tube -whereby the output signal of said oscillator4remains constant.

12. A constant output oscillator comprising, in combination,l anelectron tube including an anode, a cathode and a control grid, aVsource of potential, a tuned circuit comprising an inductor and acapacitor connected in parallel. relation, a variable resistanceelement, said anode being connected. to one Iterminal of said tunedcircuit, the second terminal of said tuned circuit being connected tovone` terminal of said variable resistance element, the second terminalof said variable resistance element being connectedto saidisourceof'potential, circuit meansl connec'ted to said control grid to sustainoscillations in said tuned circuit, rectifying means connected to saidanode to establishV a direct voltage of magnitude proportional to themagnitude of oscillations appearing at said anode, and means undercontrol of said rectified voltage to vary the resistance of saidresistance element whereby the output signal of said oscillator 'remainsconstant. 13. The combination in -accordance with claim 12 wherein saidresistance element comprises an electron tube having at least an anode,a cathode fand a control grid, .the resistance ibetween said cathode andsaid anode constituting said resistance element, and means to apply saidrectified voltage to the control grid of said vacuum tube to regulatethe current yflow therethrough.

14. 'Ihe combination in accordance with claim 12 wherein said rectifyingmeans comprises a voltage doubling rectifier circuit.

15. f` An oscillatorcomprising, in combination, a first electron tubehaving an anode, a cathode and at least two grids, a source ofpotential, a first tuned circuit comprising an inductor and a capacitorconnected in parallel relation, a second tuned circuit comprising aninductor and a capacitor connected in parallel relation', a secondelectron tube having an anode, a cathode and a control grid, the anodeof said second tube being connected to the positive terminal of saidsource of potential, the cathode of said second tube being connected toone terminal of said first tuned circuit, the second terminal of saidlfirst tuned circuit being connected to the anode of said first tube,circuit means connected to the second of said grids to sustainoscillations in said first tuned circuit, means connecting said secondtuned circuit between the first of said grids in said first tube and thecathode of said second tube, voltage doubling rectifying means connectedbetween the anode of said 4first tube and the negative terminal of saidsource of potential, and amplifying means energize-d by the rectifiedvoltage, the output of said amplifying means being applied to thecontrol grid of said second tube.

16. A mass spectrometer comprising, in combination; a gas impermeableenvelope enclosing an electron emitting filament, an ionization chamberspaced from said lament, a screen electrode positioned between said'filament and said chamber, an accelerating electrode spaced betweensaid filament and said chamber, a collector plate spaced from saidchamber, and .a plurality of groups of grids spaced between said chamberand said plate, each of said groups comprising three equally spacedgrids, the spacings between adjacent groups being n -O.3l83- (2s|t)inches, where n is an integral number, s is the spacing between adjacentgrids in each group, and t is the thickness of each grid, all of saiddimensions being in inches; lfirst and second spaced grids positionedbetween said collector plate and the group of said grids adjacent saidcollector plate; a source of voltage applied across said filament, meansto measure the current flow from said filament, and means responsive tosaid means to measure current to apply a potential to said screenelectrode to maintain the electron flow into said ionization chamberconstant, an oscillator tuned to a first frequency, a second oscillatortuned to a second lower frequency, means to modulate the output of saidfirst oscillator by the output of said second oscillator, meansconnecting one output terminal of said first oscillator to the centergrid in each of said groups, and means connecting the second outputterminal of said first oscillator to a point of reference potential;means to apply potentials to the two outside grids in each of saidgroups of polarity opposite the polarity of the ions being analyzed;means to apply a potential of polarity opposite the polarity of the ionsbeing analyzed to said first grid; means to apply a potential ofpolarity the same as the polarity of the ions being analyzed to saidsecond grid; an amplifier tuned to the frequency of Said secondoscillator, said amplifier including an adjustable feedback network, theinput of said amplifier being connectedvto said collector plate; meansto establish a first voltage of magnitude proportional to the output ofsaid amplifier, a reference voltage, a servo amplifier to amplify anydifference between said first voltage and said reference voltage; aservo motor coupled to' said feedback network, said servo motor beingactuated by the output of said servo amplifier to vary said feedbacknetwork until said voltage difference is zero; and means to measurerotation of said servo motor.

17. The combination in accordance with claim 3 wherein said potentialdividing network comprises first and second terminals across which saidsource of potential is applied; a first switch having a first movablearm and rst, second, and third contacts engageable selectively by saidfirst arm, said first cont-act being connected to said first terminal; afirst resistor connected between said second contact and said rstterminal; a second resistor connected between vsaid third contact andsaid first -terminal; a second switch having a second movable arm andfourth, fifth, and sixth contacts engageable selectively by `said secondarm, said second arm being connected to said second terminal; a firstvariable resistor having one terminal connected to said first switcharm; a second van'- able resistor having one terminal connected to saidfourth contact; a third resistor connected between said one termin-a1 ofsaid second variable resistor and said fifth contact; a fourth resistorconnected between said one terminal of said second variable resistor andsaid sixth contact; a third variable resistor; means connecting oneterminal of said third variable resistor to the second terminal of saidfirst variable resistor; a fourth variable resistor connected betweenthe second terminals of said second and third variable resistor;potential dividing means connected between the second terminal of saidfirst variable resistor and the second terminal of said third variableresistor; means to vary said first and' second switch arms in imison sothat said first switch arm is in contact with said first contact whensaid second switch arm is in contact with said sixth contact, said firstswitch arm is in contact with said second contact when said secondswitch arm is in contact with said fifth contact, and said first switcharm is in contact with said third contact when said second switch arm isin contact with said fourth contact; means to vary said first and secondvariable resistors simultaneously; and means to vary said third andfourth variable resistors simultaneously. t

18. A potential dividing network comprising first and second terminals;a source of potential applied across said terminals; a first switchhaving a first movable arm and first, second, and third contactsengageable selectively by said first arm, said first contact beingconnected to said first terminal; a first resistor connected betweensaid second contact and said first terminal; a second resistor connectedbetween said third contact and said first terminal; a second switchhaving a second movable arm and fourth, fifth, and sixth contactsengageable selectively by said second arm, said second arm beingconnected to said second terminal; a first variable -resistor having oneterminal connected to said first switch arm; a second variable resistorhaving one terminal connected to said fourth contact; a third resistorconnected between said one terminal of said second variable resistor andsaid fifth contact; a fourth resistor connected between said oneterminal of said second Variable resistor and said sixth contact; athird variable resistor; means connecting one terminal of said thirdvariable resistor to the second terminal of said first variableresistor; a fourth variable resistor connected between the secondterminals of said second and third variable resistors; potentialdividing means connected between the second terminal of said firstvariable resistor and the second terminal of said third variableresistor; means to vary said rst and second switch arms in unison sothat said first switch arm is in contact with said first contact whensaid second switch arm is in contact with said sixth contact, saidrstswitch arm is in contact with said second contact when said secondswitch arm is in contact with said ifth contact, and said rst switch armis in contact with said third contact when said second switch arm is incontact with said fourth contact; means to varyrsaid rst and secondvariable resistors simultaneously; and means to vary said third andfourth variable resistors simultaneously. p Y

19. The combination in accordance with claim 1 ,wherein said firstsource of alternating potential provides an output signal having asinusoidal wave from, and wherein said second source of potentialprovides an output signal having a square wave form.

Lederer May 8, 1934 2,068,112 Rust Jan. 19, 1937 Travis Dec, 13, 1938.24 y2,369,030 Edwards s Fe b`6; 19.45 2,400,557 Lawlor May 21,`2,449,072 Houghton ..1 Septr14, 1'948 2,487,279 Stalhane Nov. f8, 19492,535,032 Bennett Dec. 26, 1950 2,563,626 Stein et al. `Aug. 7, v191512,598,478 Worchester May 27, 1952 2,598,734 Washburn June 3,19522,602,898 Inghrarn et al. July 8, 19,52 '2,617,843 Houghton Nov. 11,`1952 2,790,945 Chop'e Apr. 30, 1957 2,854,629 Thirup Sept. 30, 195

OTHER REFERENCES Pearson: Thermistors, Their vCharacteristics and Uses,Bell Laboratories Record, December 1940, pages 106 to 111.

Bennett, Journal of Applied Physics,

